The modem printing industry primarily employs a planographic method of printing. In this type of process, the image and non-image areas of the printing plates are on the same plane of a thin metal plate. In older processes, plates were provided with variations on their surface, such as depressions or raised sections, to distinguish between the image and non-image areas. In contrast, the planographic method maintains the distinction by chemical means. To make a plate, one begins with a metal plate having a photo polymer coating which is presensitized to UV light. The plate and a piece of film with an image on it are exposed to UV light such that the photo polymer under the image is hardened according to the design of the image. The hardened parts of the photo polymer remain on the plate while the other parts are removed by known chemical processes. The hardened parts which attract ink become the image areas, and the other parts which repel ink become the non-image areas. These areas are commonly said to be "ink-loving" and "water-loving."
In printing color images, the colors are separated into four colors by a color scanner--namely, cyan, magenta, yellow and black. Other colors are produced through various combinations of these four colors. A separate printing plate must be produced for each of the four colors. In the printing process, the paper is sequentially printed by each of the four plates in an overlapping relationship to produce an image which colorfully reflects or simulates an original continuous tone image. Since a separate plate must be produced for each color, it is critical that the images on the plates be properly positioned so that the printed product will have sharp, clear images.
A conventional process for printing periodicals or the like is begun by producing a relatively large piece of film called an imposition flat. The imposition flat is similar in size to the printing plate and is ultimately used to expose the plate as discussed above. The plates are used to print forms. A form is a large sheet of paper with the pages positioned on it such that once printed, they can be trimmed and folded into pages of a publication. A four page form 10 as illustrated in FIG. 1 will be used for ease of explanation. However, printing is done with anywhere from two to sixty four pages on each side of a sheet of paper defining a form. Producing the large imposition flat used to create the form has heretofore been an exacting process. An imposition flat consists of a number of pages of the finished printed product laid out in the configuration desired for the form. The size and configuration of the layout or imposition flat is determined in a planning stage when the number of pages to be printed is decided.
FIG. 2A is flow-chart illustrating a conventional process for producing printing plates. For producing page 1 of the form 10, one first inputs a color transparency of an original color image into a color scanner. The color scanner separates the image into the four colors, corrects the colors and adjusts them for printing conditions such as inks, paper, and tonal range. The scanner communicates electronically with an image setter to output a piece of film with four images--one in each of the separated and corrected colors--of the inputted transparency. An image setter images the color separations onto the film by a laser. The film is mounted on an imaging drum which spins the film at a rapid rate while the color scanner instructs a laser head to turn on and off. The laser in affect operates to "draw" the image on the film.
The film, which is substantially the size of the surface area of the drum, is typically attached to the drum by a lead edge pin and a vacuum system. The pins on the drum are typically aligned along a lead line on the drum. Pin holes are formed along the lead edge of the film to correspond to the position of the pins on the drum. The lead edge pin holes in the film function to attach the film to the drum and thus draw the film through the image setter. The drum also has machined grooves which serve as vacuum channels. The channels are commonly machined to be located around the periphery of the mounted film. The channels have small holes which are part of a vacuum distribution system. When the image setter is activated, a vacuum is drawn through the holes in the channels to securely hold the film to the surface of the drum.
The text for page 1 is output to another piece of film 53 (FIG. 9) using a text output device such as a Linotype 300 manufactured by The Linotype Co. of Hauppauge, N.Y.
Once the separated images are on the film, one image per color, the film is cut apart 50a-50d and 51a-51d (FIG. 8) for registration and assembly. Registration and assembly are manual processes which require (1) the image films to be stripped in register onto carriers for assembly, and (2) the assembled images to be exposed to a specific spot on a larger piece of film to form an imposition flat.
Stripping the image films in register entails the manual positioning and taping of the image film and text film to mylar carriers 52a-52d, 54 (FIGS. 8-10). Each of the mylar carriers have pin holes 55 and 56 along the top and bottom edges of the sheets, respectively. Registration of the films is accomplished by use of the pin holes 55, 56 in the mylar sheets and pins 41 fixed on a light table 40.
Referring to the assemblies of FIGS. 9 and 10, the image films 50a-50d, 51a-51d and the text film 53 for page 1 are taken to the light table 40 onto which a sheet of mylar 52a has been placed with pin holes 55 and 56 received over pins 41 (FIG. 11). The image films 50a, 51a for one color, for example cyan, are attached by tape 57 to the mylar 52a in the proper positions for the page. In the drawings, two spaced apart images are illustrated in position over a text portion. This arrangement is merely exemplary. The orientation of the images and text of course varies from page to page. After the proper placement of the images of one color, another sheet of mylar 52b is put on top of the first assembly 50a, 51a, 52a, using pin holes 55 and 56 (FIG. 11). The image films 50b, 51b for the next color, for example magenta, are visually positioned in exact register to the first 50a, 51a and then manually taped to the second sheet of mylar 52b. The second sheet of mylar 52b is then removed, and a third sheet of mylar 52c is put on top of the first assembly 50a, 51a, 52a with pin holes 55 and 56 received over the pins 41. The third image films 50c, 51c for the next color, for example yellow, are also visually positioned onto the third sheet of mylar 52c, and manually taped to it. These steps are repeated to properly position and tape the fourth image films 50d, 51d onto the fourth sheet of mylar 52d. The text film 53 is also visually positioned, in a desired location, onto a sheet of mylar 54 and taped. Since the mylar carriers 52a-52d and 54 were all engaged to the same set of pins on the light table, the image films 50a-50d, 51a-51d and 53 are in register with respect to the pin holes 55 and 56 in the mylar carriers.
The steps of scanning, separating, imaging, visual positioning and taping onto carriers must be repeated for each page of the form. As a result, the production of a four page form requires a total of twenty mylar sheets, four color separations and one text for each page. All of the mylar sheets for one page will be in register with respect to the pin holes.
Once the image films for the form are in register on the mylar carriers they are imposed on a larger piece of film or directly onto a plate. One imposition process involves a step and repeat machine. All mylar carriers having the image films of one color, for example cyan, for each of the four pages are hung in turn on a camera chase by the pin holes in the mylar carriers. A larger piece of film or a plate are usually held on the machine with a vacuum during exposure. The image films are exposed to the larger piece of film or directly to the plate at a predetermined positions to create an imposition flat or printing plate. The step and repeat machine uses a coordinate system to locate the positions for the exposures. The resulting imposition flat (FIG. 3) has all four pages on it for the one color, in this case cyan. Likewise, all mylar carriers having the image films of another color, for example magenta, for each of the four pages are also hung in turn on a camera chase by the pin holes in the mylar carriers, and exposed to a larger piece of film or the plate at predetermined positions. The resulting imposition flat or printing plate has all four pages on it for magenta. The same steps are repeated for the yellow images, the black images, and the text films.
If imposition was done directly onto the plates, the plates are ready to be used in conventional offset lithography. If imposition flats are made for each color and for the text, the flats are used to produce the metal printing plates (FIG. 4) for each respective color. For example, the cyan imposition flat is used to expose a printing plate which is processed and thereby results in a plate which will print the color cyan onto the paper. The magenta imposition flat is used to expose a printing plate and produce the plate which will print the color magenta on the paper, and so on. The printing plates can then used in conventional offset lithography. Versions of the step and repeat machines having various degrees of automation are known.
As can be appreciated, the printing process is highly manual. In particular, stripping the image films in register on a light table is very repetitious and time consuming.
Another conventional method of producing printing plates is illustrated in the flow-chart in FIG. 2B. In this method, all of the pages of a form must be planned and laid out before commencing. The color images and text for all of the pages of the form are input to the color scanner. The color scanner separates the images into the four colors.
Typically a large piece of film with pin holes on its leading edge is attached to a correspondingly large drum of an image setter. The image setter is instructed by the color scanner to image the large piece of film corresponding to the form, producing a film form, for each color. The imaging is generally done one color at a time. In other words, the film form that comes out of the image setter will have the images and text for all of the pages of a form. Four of these film forms will be produced, one for each color: cyan, magenta, yellow and black with the text on it. Each of the pages will have the images and text in position and register with respect to themselves. The pin holes provide a registry system for the entire form or block of pages but not the individual pages.
In the production of periodicals, the film form must still ordinarily be cut apart for the imposition step. The individual films must still be put on mylar carriers having pin registry holes which correspond to the pins used to create an imposition flat. A particular color film must be placed on a mylar carrier in position and register to the other color films for that page. The process is similar to the stripping done in the previously discussed prior art method, although it is somewhat simplified since all the images for a color on a particular page and the text are in register with each other on one piece of film. Yet, until the pages are positioned and attached to the mylar carriers, the different colors are not in register with each other. Hence, although the film is imaged with all page elements, the manual assembly on carriers is still required.
A few book publishers have developed specialized apparatus for creating imposition flats directly from the imaged film; that is, without the mylar carriers. The imposition apparatus is constructed to support the entire eight page films in register with one another. The lead holes on the film are used to align the different colors in register. This method, however, requires all of the pages for the film form to be laid out and ready to print before the pages are scanned or imaged. Since eight consecutive pages are imaged to the same film which is directly used to make the printing sheet, the process fails to accommodate the insertion of advertisement pages which are frequently available only just before printing. As a result, no time is generally available to digitalize the advertisement and print the film in time to make the mailing deadlines. Consequently, this process is limited to some types of book publishing, and is not practical for use in the periodical printing industry.